1. Field of the Invention
This invention relates to a device which performs auto-zooming that is particularly useful in video cameras.
2. Description of the Related Art
When shooting footage with a video camera equipped with a zoom function, if the object is stationary, as in the case of scenery or still life, it is unnecessary to change the focal length once it is set. However, if the object is mobile, it is necessary to continuously correct the focal length according to the movement of the object in order to keep the size of the object image consistent in all shooting situations. When the object moves away from the camera, the size of the object image becomes small even if the focal length was originally set to the appropriate object size, which necessitates readjustment of the focal length in a telescopic direction. Conversely, when the object moves closer to the camera, the object image increases in size, which necessitates readjustment of the focal length in a wide-angle direction. In short, it is necessary to repeat zooming frequently according to the movement of the object.
There exist conventional video cameras which perform zooming via a manual operation. Such video cameras are inconvenient in that, as described above, zooming must be performed frequently when filming a mobile object.
It is therefore desirable to make auto-zooming available.
However, in case of auto-zooming, the following problems exist:
1) If the zoom speed is fixed, it cannot follow a rapidly-moving object; while if the object movement is slow, the zooming operation becomes jerky, resulting in an undesirable screen image. PA1 2) When a conversion lens used to change the focal length is separately mountable, the focal length of the camera itself must be set differently depending on whether or not the conversion lens is mounted. PA1 3) Where the camera has a pause function which enables it to temporarily stop shooting, and another object passes across the camera while it is pausing, the automatic zooming function zooms in on the other object, using a different focal length than that appropriate for the original object. Therefore, when the pause button is released, the focal length may be inappropriate. PA1 4) In the case of an inner-focus or rear-focus type optical system, even where the object distance remains the same, performing a zooming operation creates a discrepancy regarding the location of the focusing lens. (In the case of an optical system which performs focusing by moving the front lens, even if zooming is performed, the location of the front lens does not change so long as the object distance remains the same.)
Moreover, in such optical systems, when the optical system is set to the wide-angle setting through zooming, it is difficult to obtain accurate information as to the object distance because the focusing lens changes its location only within an extremely small range (see FIG. 20), and when the optical system is set to the wide-angle setting, the field depth is large. Because of this, if the object distance is to be determined based on the location of the focusing lens, and zooming is to be automatically performed according to the object distance thereby determined, it is necessary to use an extremely precise sensor to accurately determine the location of the focusing lens.